Reduction of explosive hazard in nonfloating roof type storage tanks in which combustible liquids are stored



Dec. 15, 1959 2,917,201

H. L.- DRAPER REDUCTION OF EXPLOSIVE HAZARD IN NONFLOATING ROOF TYPESTORAGE TANKS IN WHICH COMBUSTIBLE LIQUIDS ARE STORED 2 Sheets-Sheet 1Filed June 24. 1957 AIR INVENTOR. H.L. DRAPER FIG. BY: Z

A T TORNEVS 2 Sheets-Sheet 2 /IOI / )4 REuEF VALVE SETTING VACUUMBREAKER SETTING H. L. DRAPER REDUCTION OF EXPLOSIVE HAZARD INNONFLOATING ROOF TYPE STORAGE TANKS IN WHICH COMBUSTIBLE LIQUIDS ARESTORED so so TANK TEMPERATURE, F

EXPLOSIVE DANGER ZONE Dec. 15,

Filed June 24. 1957 United States Patent Ofice 2,917,201 Patented Dec.15, 1959 REDUCTION OF EXPLOSIVE HAZARD IN NON- FLOATING ROOF TYPESTORAGE TANKS IN WHICH COMBUSTIBLE LIQUIDS ARE STORED Homer L. Draper,Bartlesville, kla., assignor to Phillips Petroleum Company, acorporation of Delaware Application June 24, 1957, Serial No. 667,605

7 Claims. (01. zzo-ss -Th-is invention relates to a method and apparatusfor reducing. the explosive hazard in nonfioating roof tanks in whichcombustible liquids, the vapors of which form explosive mixtures withair, are stored. In one aspect itrelates to a method and apparatus forreducing the explosive hazard 'in a nonfloating roof type storage tankin which petroleum or petroleum products are stored.

This explosive hazard problem is solved, in many instances, byinstallation of floating roof type tanks. Such tanks are common andfurther description thereof need not be made. However, many tanks,particularly older tanks, were constructed with nonfloating roofs. Whena petroleum material is withdrawn from the tank and displacedwith air,an explosive mixture may remain in the tank. Such tanks ordinarily havesample hatches and breather vents and it is through these openings thatair enters the tanks upon removal of the liquid contents. Furthermore,breathing of tanks admits air into the vapor space. Regardless of howthe air enters the tanks, whether it is by breathing or by removal ofthe liquid contents, a hazardous condition may exist.

The object of my invention is to provide apparatus and a method toeliminate explosive hazard conditions in nonfloating roof type storagetanks used for the storage of liquids whose vapors form explosivemixtures with air.

In tanks which are provided with sample hatches and breather tubes, atcertain amount of liquid contents of the tank is lost by vaporization.My method and apparatus is applicable to tanks which are not providedwith conventional sample hatches and/ or breather tubes. In other words,my invention is applicable to vaportight tanks." The use of vapor-tighttanks has an added advantage that vaporization of product with itssubsequentloss is largely eliminated. The explosive conditions of thevapor contents of the tank, according to my invenform, and partly insection, an arrangement of apparatusparts suitable for use in practicingmy invention.

Figure 2 is aplot of tank pressure versus tank temperature illustratingthe principle upon which my invention is based.

Figure? illustrates, schematically, one form of a portion of theapparatus of Figure 1.

. Myinvention is directed to an apparatus for eliminating the explosivehazard sometimes present in the vaporcontaining space of a nonfioatingroof type tank used for the storage of liquids, the vapors of which formexplosive mixtures with air, comprising, in combination, a fluid-tight.tank, means for inlet and outlet of liquid from said tank, a firstconduit communicating with said storage tank for venting vapors, avacuum relief valve communicating with said tank, a motor oper- Itroller 14.

ated pop valve assembly in said first conduit for regulation of vaporventing, atemperature responsive device in the normally vapor containingspace. of said tank, a temperature responsive controller in operativecommunication with said temperature responsive device, a reset pressurecontroller in operative communication with the motor of said motoroperated pop valve assembly and with said temperature responsivecontroller whereby said reset pressure controller regulates fluidpressure to said motor in response to temperature indicated by saidtemperature responsive device. V

The method of my'invention involves a method for maintaining vaporouscontents of a non-floating roof fluid-tight zone free from explosivehazard, a combustible liquid, the vapors of which form explosivemixtures with air, being disposed in said zone, comprising maintaining amixture of said vapors and air in said zone at a volume ratio of air tovapors below the ratio of air to vapors which form an explosive mixturebyregulating the pressure of the mixture of said vapors and air in saidzone in response to the temperature of the mixture of vapors and air insaid zone.-

My invention is applicable to the storage of liquids having Reid vaporpressures at F. less than atmospheric pressure such that uponwithdrawing the liquid from the storage vessel a vacuum valve operatesadmitting air thereby setting up possible explosive conditions.

The term Reid vapor pressure, or RVP, as used throughout thisspecification and claims, is a measure of the volatility of a nonviscouspetroleum product in terms of pounds per square inch at 100? F. asdetermined by A.S.T.M. method designated as D323-37T.

Referring now to the drawing and specifically to Figure 1, referencenumeral 10' identifies a nonfloating.

A pipe 19. is provided for venting vapors from the tank in response tooperation of the control apparatus of my.

invention. A thermocouple 12 is illustrated as extending through theroof of tank 10 into the space normally occupied by vapor. Thethermocouple can, however, be inserted through the side wall of the tankprovided it extends into. the space normally occupied by vapor.

Thermocouple 12 is connected operatively with a tem perature responsivecontroller 13. A reset flow controller =14 is connected operatively withthe temperature responsive controller 13 and with a normally closedmotor operated relief valve 15. A pipe 17 conducts air under pressure,from a source not shown, to the reset. pressure controller while a pipe18 conducts air of regu lated pressure from the reset pressurecontroller 14 to the underside of a diaphragm 24 of the motor of thevalve 15. As mentioned, this valve is a normally closed valve,

the valve being held in a normally closed position by acompressionspring 22 arranged as illustrated. A vent.

tube 20 is provided for venting of air in case of leakage throughdiaphragm 24 and to maintain atmospheric pressure above the diaphragm.An air bleed 21 is provided forbleeding of a small flow of air from theunderside of the diaphragm. Reference numeral 23 identifies a re setpointer in conjunction with the reset pressure con- In many operationssuch reset pointers as pointer 23 are set manually for maintenance ofconstant outlet pressure in tubes corresponding to tubes or pipes 18;but, according to my invention, the reset pointer is continuously resetby operation of the temperature responsive controller 13. Temperaturecontroller .13, as n 3 V illustrated in Figure 3, includes a coil inwhich is disposed a movable rod 31 pivoted to a baffle 34 and biaseddownward by tension spring 32. Amplifier 39 amplifies the very smallsignal set up by thermocouple 12, the amplified signal moving rod 31upward in proportion to a temperature increase sensed by thethermocouple. As the rod 31 moves upward, the arrowed end of baffle 34moves downward on resistor 33 to decrease to a coil in the resetcontroller 14. Rod 35 in the reset controller coil then moves downwardunder the influence of decreased in the reset controller coil andtension spring 38 to bias valve 37 toward a closed position. When valve37 moves toward a closed position, less pressure air from line 17 entersthe reset controller 14 and thence pressure via line 18 to the underside of diaphragm 24 is reduced. Upon sensing a decrease in temperaturein tank 10 by thermocouple these several elements operate in a mannerjust the reverse to that just described to open valve 37 and increasethe air pressure on the under side of diaphragm 24. Accordingly, thetemperature responsive controller 13 operates in response to thetemperature as indicated by thermocouple 12 and this controller thencontinuously resets pointer 23 of the reset pressure controller and thiscontinuous resetting continuously regulates How of air under pressurethrough pipe 18 to regulate the air pressure on the underside ofdiaphragm 24.

A valve head 25 is illustrated in Figure 1 as being provided with a seat26 in such a manner that as gas pressure is exerted in pipe 19, thispressure is also exerted on the underside of valve head 25 to assist inopening the valve against the compression of compression spring 22. Theopening of this valve is occasioned by the sum of two forces, one forceacting upward on the underside of valve head 25 and the other force isthe air under pressure from pipe 18 acting on the underside of diaphragm24. In the particular example to be described hereinbelow, thecompression spring is so adjusted that-a total pressure of 20 pounds persquare inch in tank 10 acting on the underside of valve head 25 in theabsence of air pressure below the diaphragm opens the valve against thecompression spring. Accordingly, when pressure is exerted upward andagainst the underside of diaphragm 24, less pressure than 20 pounds persquare inch is required on the underside of valve head 25 to raise thevalve against the compression of spring 22. I

Referring now to Figure 2, which illustrates the principle upon which myinvention operates, line 100 is intended to be a plot of the values ofpressure versus temperature under which conditions a mixture of vaporsof a five-pound Reid vapor pressure gasoline and air will not explodebecause the mixture is too lean in hydrocarbon vapor or too rich in airunder the pressures and temperatures illustrated by line 100. In otherwords, line 100 represents the lower or lean explosive limit with air ofa five-pound Reid vapor pressure gasoline. To the left of this line forthis particular product the ratio of air to vapor is too great to beexplosive.

Line 101 represents the upper or rich explosive limit and to theright-hand side of this line the ratio of air to vapor is too low to beexplosive, or, in other words, the mixture is too rich in hydrocarbonvapors to be explosive. Line 101 represents the rich or upper explosivelimit. The 20 p.s.i.g. (pressure per "square inch gauge) line 102represents the maximum relief valve setting. The p.s.i.g. line 103,represents atmospheric pressure, that is, it is the pressure at whichthe vacuum breaker will operate to admit atmospheric air into the tankwhen the pressure in the tank is below that of the atmosphere. The tankpressure is intended not to be below this value because of the operationof the vacuum breaker. The shaded area, bounded by lines 103, 101, 102and 100 (if this latter line were extended to pressures below about 27p.s.i.g. and temperatures below 0 F.), represents thetemperature-pressure region for the 5.0 Reid vapor pressure gasoline ofthe specific example in which explosive mixtures exist.

According to Figure 2, with the motor relief valve of Figure 1 being setat 20 pounds pressure, such a quantity of a five-pound Reid vaporpressure gasoline is contained in tank 10 that the total fluid pressurein the tank is approximately five pounds gauge pressure. If the tank andits contents are at 40 F., this pressure is indicated on the 40 F.temperature line by point D. Upon pumping an additional quantity of thesame gasoline at 40 F. into the tank, the pressure in the tank rises andwill follow the 40 F. temperature line upward until such a time as pointC is reached. This point C is a point on a dotted line 104 which, asindicated in Figure 2, is positioned on the righthand side of line 101,that is, it is positioned in the area which is too rich in hydrocarbonvapors to be explosive. The temperature responsive controller 13 ofFigure 1 still operates when point C is reached, having at point D resetcontroller 23of the reset pressure controller 14 to admit sufiicient airfrom pipe 17 through pipe 18 to the underside of diaphragm 24 to openthe valve 25 and release pressure from the tank when the 9 /2 poundspressure in the tank is reached at point C. In other words, if valve 25did not open at this time, upon further pumping of liquid gasoline intothe tank the pressure would follow the 40 F. line upward and ultimatelyreach a value of 20 pounds at which pressure spring 22 will allow thevalve to open. However, from approximately a 9 /2 pounds pressure in thetank up to 20 pounds, the mixture of gasoline vapors and air is withinthe explosive range (shaded area) and obviously an explosive hazardexists. But, by operation of the temperature responsive controller toreset the reset controller, vapor is vented from the tank when apressure of approximately 9 /2 pounds per square inch is reached in thetank so that vapor is vented from the tank to maintain the pressure inthe tank at a maximum of about 9 /2 p.s.i.g. as long as the temperatureremains at 40 F.,- and this 9 /2 p.s.i.g. is outside the explosiverange.

Again, if the tank contains such a volume of the abovementioned gasolineat 40 F. that the pressure in the tank is approximately 5 p.s.i.g., asindicated by point D in Figure 2, this point lies in the explosion-freearea to the right-hand side of line 101. Upon increase of temperaturewithin the vapor space of the tank from 40 F. to about 70 F., pressureincreases, and a plot of temperature versus pressure follows line 107.In this case an explosion-free mixture of vapor still exists in the tankbecause line 107 did not enter the hatched danger zone area of thediagram.

However, if, for example, tank 10 contains such an amount of theabove-mentioned gasoline that pressure in the tank is about 5 p.s.i.g.,which condition is identified by point D in Figure 2, and upon pumpingin of a quantity of this same gasoline at a temperature above 40 F., thetemperature and pressure of the vaporous contents of the tank may beconsidered to follow line 110 if the tank were not equipped with mysafety vent device. According to my invention, when the vapor contentsof the tank as regards temperature and pressure reach point G, thetemperature responsive controller 13 operates to reset pointer 23 of thereset controller which, in turn, operates to admit only sufficient airthrough pipe 18 to the underside of diaphragm 24 to operate inconjunction with the pressure acting on the underside of valve head 25to open this'valve and vent vaporous contents of the tank. In thismanner, with valve head 25 opened at point G, pressure in the tank doesnot rise above approximately 11 p.s.ig. as long as temperature remainsat about 45 F.; and, accordingly, the vapor contents of the tank are notwithin explosive limits. If applicants invention were not used in thislatter case and warm gasoline were continued to be pumped into the tank,the temperature and pressure conditions would continue to follow line110 until point F were reached under which condition valve 25 would notopen until 20 p.s.i.g. (point F) were reached in the tank, and point Frepresents an explosive mixture. In thetwo cases explained hereinabovein which gasoline was pumped into the tank, if the temperature in thetank does not deviate from 40 F., the pop valve stays open and ventsvapors at about 9 /2 p.s.i.g. continuously as long as liquid is pumpedinto the tank. If warm gasoline is pumped 'into the tank, as mentionedabove, the vaporous contents of the tank reach point G and the valvethen opens to vent vapors from the tank; but, upon continued'pumping ofwarm gasoline into the tank, temperatureof the vapors increasesand sincethe valve is open, the vapor contents of the tank cease to follow line110 into'the explosive. danger zone and follow line 104 upward andto theright outside of the explosive danger zone until such time as liquid isno longer pumped into the tank. Thus,jin this latter case, the more warmgasoline pumped into the tank the better the reset controller apparatusof my invention operates to maintain a higher pressure in the tankcorresponding to the temperature of the vapor in the tank.

The apparatus of my invention operates equally as well upon reduction ofthe temperature in the tank and upon withdrawal of liquid therefrom. Forexample, if the tank were filled withthe above-mentioned five-pound Reidvapor pressure gasoline at 70 R, and since the pressure relief valve isset to open at 20 p.s.i.g., point A illustrates this set of conditions.If, for example, a cool thundershower drops water upon the top of thetank, it is obvious that the temperature therein will be lowered andline106 is intended to illustrate the temperature of the vapor.containing portion of the tank as occasioned by cooling and partialcondensing of the vapor without removal of any liquid from the tank.Under these condi tions, if cooling continued to 40 F., pressure wouldalso decrease and finally point B would be reached. Point B lies withinthe explosivearea. According to my invention, upon cooling of thevaporcontents of the tank, the temperaturedecreas'es with a simultaneousdecrease of pressure following line 106 until such time as line 106intersects line 104 at'point B. At this point the tem peratureresponsive controller-reset pressure controller of my invention operatesto vent vapors from the tank so that further cooling will cause apressure reduction and line 104 will befollowed outside the explosivearea until such time as temperature ceases to decrease. If the reliefvalve 15 did not open upon further cooling of the vapors in the tank,point B would be reached when the temperature of the vapors reached 40F. instead-of pointC. J

. Beginning at point A, i.e., with the tank containing 7.10 F. gasolineof five pounds RVP at 20 p.s.i.g., if liquid'jcontents .are'removed fromthe tankalong with simultaneo'us cooling of th'e'vapor contents of thetank, the pressure-temperature conditions of the vapor in the tank mayfollow line 108. Upon continued following of line 108 it is noted thatthis dotted line does not enter the explosive danger zone until pressureis reduced to a value below about 11 /2 p.s.i.g. at about 45 F. Thus thepop valve need not relieve pressure of the tank unless temperature isreduced to a value below about 45 F. A reduction of pressure only keepthe condition on the rich side of line 101. i

Still another condition is that if liquid at 70 F. were withdrawn fromthe tank without a simultaneous decrease of the temperature of thevapors in the tank, and beginning at point A pressure in the tank fallsand follows line 109 downward. Line 109, as will be seen from Figure 2,is also outside the explosive range. Thus the tank may be entirelyemptied of liquid at 70 F. without ever entering the explosive regionprovided the temperature of the remaining vapors remains at 70 F.

The above example involving a five-pound Reid vapor pressure gasolinewas given merely as an example of the operation of the apparatus andmethod of my invention. The diagram of Figure 2 isfa diagrammaticrepresentation of the pressure and temperature conditions relative toexplosive mixtures with air of a five-pound Reid vapor pressuregasoline. Other gasolines, oils, liquid hydrocarbons such as pentane, orhexane or other liquids such as alcohols, acetone, or the like, whosevapors form explosive mixtures with air, can be stored inanexplosionfree condition according to my invention. However, with each ofthese other liquids to be stored a separate and individual explosivedanger zone diagram will need to be provided because explosive limits ofother combustible liquids are different.

While I have described my invention as employing a relief valve 15 setat 20 p.s.i.g., it is realized by those skilled in such art that thevalve can be set to open at pressures above or below 20 p.s.i.g. asdictated by local conditions. If the safety apparatus is applied me highpressure tank, obviously a higher pressure can be maintained. If a tankis used which will safetly maintain a pressure of only 10 p.s.i.g., thenthe relief valve should 1. An apparatus for eliminating the explosivehazard sometimes present in the vapor containing space .of afluid-tighttank used for the storage 'of a liquid, the vapors of which formexplosive mixtures with air, comprising, in combination, a fluid-tighttank, means for inlet. and outlet'of liquid from said tank, a conduitcommun'ieating with said tank for venting vapors, a vacuum relief valvecommunicating with said tank, a pressure re lief motor valve in saidconduit for regulation of venting of vapors from said tank, the motor ofthis latter valve being a fluid pressure operative motor, means biasingsaid pressure relief valve normally closed, a temperature s'ens' ingdevice in the normally vapor containing space of said tank, atemperature responsive controller in operative communication with saidtemperature sensing device, a reset pressure controller inoperativecommunication with said motor and with'said temperatureresponsive controller, said reset pressure controller being adapted toreset fluid pressure to said motor in response to an increase intemperature in said tank as sensed ,by said sensing device whereby saidmotor and the pressure 'of said vapor in said tank cooperate to opensaid pressure relief valve.

against the means biasing said valve closed.

2. The apparatus'of claim 1 wherein'said temperature sensing device is athermocouple.

3. A method for maintaining the vaporous contents of a fluid-tight zonefor storage of liquid free from explosive hazard, a combustible liquid,the vapors of which form explosive mixtures with air, being disposed insaid zone, and the vapor pressure of the liquid in said "zone being lessthan atmospheric pressure at storage temperature, comprising maintaininga mixture of said vapors and air in said zone at a volume ratio of airto vapors too rich to form an explosive mixture by reducing the pressureof the mixture of said vapors and air in said zone by venting vapors andair therefrom in response to a combination of the pressure of themixture of said vapors and air in said zone and of a valve actuating airpressure, the latter air pressure being inversely proportional to thetemperature of the mixture of vapors and air in said zone.

4. A method for eliminating the explosive hazard sometimes present inthe vapor containing space in a fluidtight storage zone in which isdisposed a combustible liquid, the vapors of said liquid formingexplosive mixtures with air, and the vapor pressure of the liquid storeda 7 being less than atomspheric pressure at storage tempera ture,comprising introducing said liquid into said fluidti'ght storage zone,at an ambient temperature, continuing introduction of said liquid intosaid zone until fluid pressure in said zone reaches a pressure at whicha mixture of vapors of said liquid and air forms an explosive mixture atthe temperature of vapor in said zone, main taining the mixture of saidvapors and air in said zone at a volume ratio of air to vapors belowthat ratio of air to vapors which forms an explosive mixture at saidtemperature by venting air and vapors from said zone until the pressuretherein is below the pressure at which the air and vapors remaining havesaid volume ratio, said venting being carried out in response to acombined pressure of said vapors and air in said zone and an extraneousair pressure, said extraneous air pressure being inversely proportionalto the temperature of the mixture of vapors and air in said zone.

5. An apparatus for eliminating the explosive hazard sometimes presentin the vapor containing space of a fluidtight tank used for the storageof a liquid, the vapors of which form explosive mixtures with air,comprising, in combination, a fluid-tight tank, means for inlet andoutlet of liquid from said tank, a first conduit communicating with saidstorage tank for venting vapors, a vacuum relief valve communicatingwith said tank, a spring loaded pressure relief valve in said firstconduit for regulation of flow of vapors from said tank, said springbiasing said relief valve closed, said relief valve having a valve headand a seat for said valve head, said valve head being so disposed withrespect to said seat that fluid pres-- sure from said tank tends to opensaid valve from said seat, means biasing said valve head closed againstsaid seat, a motor operatively connected with said valve head foropening said valve head from said seat, said motor being adapted to opensaid valve from said seat against said bias with the assistance of fluidpressure from said tank acting on said valve head, a temperatureresponsive device in the normally vapor containing space of said tank, atemperature responsive controller in operative communication with saidtemperature responsive device, a reset pressure controller in operativecommunication with said motor and with said temperature responsivecontroller whereby said reset pressure controller resets fluid pressureto said motor in response to temperature indicated by said temperatureresponsive device and said pressure relief valve being adapted to openagainst the bias of said spring under the combined influence of tankpressure and the reset fluid pressure to said motor.

6. An apparatus for eliminating the explosive hazard sometimes presentin the vapor containing space of a fluid-tight tank used for the storageof a liquid, the vapors of which form explosive mixtures With air,comprising, in combination, a fluid-tight tank, means for inlet andoutlet of liquid from said tank, a conduit communi- 7 a 8 eating withthe storage tank for venting vapors, a pressure relief valve in saidconduit for regulation of flow of vapors from said tank, said reliefvalve having a valve head and a seat for said valve head, said valvehead being so disposed with respect to said seat that fluid pressurefrom said tank tends to open said valve from said seat, means biasingsaid valve head closed against said seat, a motor operativelyconnectedwith said valve head for opening said valve head from said seat, saidmotor being adapted to open said valve from said seat against said biaswith the assistance of fluid pressure from said tank acting on saidvalve head, a reset pressure controller in operative communication withsaid motor, a temperature responsive controller in operativeconimunication with said reset pressure controller and means responsiveto the temperature in the vapor space of said tank to actuate saidtemperature responsive controller whereby said temperature responsivecontroller resets said reset pressure controller to regulate fluidpressue to said motor to open said valve.

7. A method for maintaining the vaporous contents of a fluid-tightstorage zone free from explosive hazard, a combustible liquid, thevapors of which form explosive mixtures with air, being disposed in saidzone, comprising providing a pressure relief valve in operativecommunication with the vapor space of said zone, said relief valve beingset to relieve vapor from within said zone at a safe fluid-tight zoneworking pressure higher than the vapor pressure of said combustibleliquid at storage temperature, a mixture of said vapors and air alsobeing present in said zone, maintaining the pressure of said mixture insaid zone at a value below that which is within the explosive pressurerange at ambient zone temperature by venting vapors and air through saidrelief valve, said relief valve being actuated for said venting byimpressing fluid pressure upon said valve in excess of said setpressure, the fluid pressure impressed on said valve being a sum of thepressure of said vapors and air of said zone and a valve actuating airpressure, said valve actuating air pressure being impressed on saidvalve in response to temperature of fluid in said storage zone, saidactuating air pressure being decreased as temperature of fluid in saidstorage zone increases, and said actuating air pressure being increasedas temperature of fluid in said storage zone decreases.

References Cited in the file of this patent UNITED STATES PATENTSColvin, Jr. et al May 22, 1956 UNITED STATES PATENT OFFICE CERTIFICATEOF CORRECTION Patent No. 2,917, 2Q1 December 15, 1959 Homer L.'Draper Itis hereby certified that error appears in the printed specification ofthe above numbered patent requiring correction and that the said LettersPatent should read as corrected below.

Column 7, lines 3 and 4, after "temperature strike out "continuingintroduction of said liquid into said zone".

Signed and sealed this 9th day of August 1960.

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Ofiicer Commissioner ofPatents

7. A METHOD FOR MAINTAINING THE VAPOROUS CONTENTS OF A FLUID-TIGHTSTORAGE ZONE FREE EXPLOSIVE HAZARD, A COMBUSTIBLE LIQUID, THE VAPORS OFWHICH FORM EXPLOSIVE MIXTURES WITH AIR, BEING DISPOSED IN SAID ZONE,COMPRISING PROVIDING A PRESSURE RELIEF VALVE IN OPERATIVE COMMUNICATIONWITH THE VAPOR SPACE OF SAID ZONE, SAID RELIEF VALVE BEING SET TORELIEVE VAPOR FROM WITHIN SAID ZONE AT A SAFE FLUID-TIGHT ZONE WORKINGPRESSUR HIGHER THAN THE VAPOR PRESSURE OF SAID COMBUSTIBLE LIQUID ATSTORAGE TEMPERATURE, A MIXTURE OF SAID VAPORS AND AIR ALSO BEING PRESENTIN SAID ZONE, MAINTAINING THE PRESSURE OF SAID MIXTURE IN SAID ZONE AT AVALUE BELOW THAT WHICH IS WITHIN THE EXPLOSIVE PRESSURE RANGE AT AMBIENTZONE TEMPERATURE BY VENTING VAPORS AND AIR THROUGH SAID RELIEF VALVE,SAID RELIEF VALVE BEING ACTUATED FOR SAID VENTING BY IMPRESSING FLUIDPRESSURE UPON SAID VALVE IN EXCESS OF SAID SET PRESSURE, THE FLUIDPRESSURE IMPRESSED ON SAID VALVE BEING A SUM OF THE PRESSURE OF SAIDVAPORS AND AIR OF SAID ZONE AND A VALVE ACTUATING AIR PRESSURE, SAIDVALVE ACTUATING AIR PRESSURE BEING IMPRESSED ON SAID VALVE IN RESPONSETO TEMPERATURE OF FLUID IN SAID STORAGE ZONE, SAID ACTUATING AIRPRESSURE BEING DECREASED AS TEMPERA-